In many applications of electronic packages, it is desirable to provide features of non-uniform widths in a substrate. For example, U.S. patent application of Anigbo et al, Ser. No. 60/009116, filed Dec. 22, 1995, U.S. Pat. No. 5,881,193 which is incorporated by reference herein, describes an optical subassembly formed on a semiconductor substrate and including a laser and a photodiode mounted on the substrate. In order to more effectively couple the laser light to the photodiode, a channel is etched into the substrate, the channel being wider in the area under the photodiode than in the area under the laser.
While such a feature could be formed using two mask levels, it would be more economical to fabricate the feature with a single mask level while controlling the transition from the narrow end to the wider end of the channel.
Another problem which exists in etching substrates is the inability to quickly and easily determine the end point of an etching operation. It is, therefore, desirable to provide features which will give a clear visual indication when the etching operation should be terminated.